Vitreous seal protector



P 1943- W.G. WAGENER 2,329,019

VITREOUS SEAL PROTECTOR Filed Oct. 6, 1941 2 Sheets-Sheet 1 IN VEN TOR W/NF/ELD a. WAGENER.

ATTORNEYS.

p 7, 1943- w. G. WAGENER 2,329,019

VITREOUS SEAL PROTECTOR Filed Oct. 6, 1941 2 Sheets-Sheet 2 INVENTOR, W/NF/ELD a. WAGENER.

Arron/VH5.

Patented Sept. 7, 1943 VITREOUS SEAL PROTECTOR Winfield G. Wagoner, Palo Alto, Calif., assignor to Heintz & Kaufman, Ltd., South San Francisco, Calif., a corporation of Nevada Application October 6, 1941, Serial No. 413,737

6 Claims.

My invention relates to vitreous seal protectors and more particularly to a means and method of protecting the seal of a lead wire through a vitreous envelope in order to prevent or greatly reduce destructive electrolysis of the envelope material adjacent the seal.

Vacuum tubes of the thermionic multi-electrode type, having vitreous lead seals, when used as self oscillators or as amplifiers of oscillating currents, have a tendency to cause failure of the grid lead. The grid lead usually operates at an average negative potential with respect to the anode, and often fails by virtue of a cracking of the vitreous portion of the envelope surrounding the gridseal, after a relatively few hundred hours of use.

An investigation of grid seals which have failed or cracked in this manner, shows that in most cases the grid seal has become discolored adjacent the point where the lead is sealed through the vitreous envelope. It is my belief that this discoloration is due to electrolysis of the vitreous material adjacent the seal of the lead though the envelope, and that this electrolysis has in some manner, not fully understood at this time, changed the physical and chemical characteristics of the vitreous material so that it no longer matches the lead in expansion characteristics. Thus, when the glass has changed sufliciently in its expansion characteristics from the lead, and while the lead is contracting or expanding due to heat conditions of operation, the seal cracks, lets air into the tube, and the tube thereupon fails. my belief that this destructive electrolysis is due to the average difference of potential between the anode lead and the grid lead, and always is most destructive at the more negative lead.

It is the main object of my invention to so protect the lead, and the seal, so that destructive electrolysis cannot take place in the vitreous material immediately adjacent the lead, and to provide means whereby the current density causing electrolysis may be greatly reduced. The electrolysis which does occur is removed entirely from the immediate vicinity of the seal so that the vitreous material surrounding the lead is not changed in its characteristics to a sufficient extent, that cracking may occur.

In the drawings:

Fig. l is a diagrammatic view, partly in section and partly in elevation, of a conventional threeand anode sealed into a vitreous envelope and It is also showing the lead protector of my invention applied to the grid lead.

Fig. 2 is a plan view of the form of lead protector applied to the tube of Fig. 1.

Fig. 3 is a plan view of an alternate form of protector, and

Fig, 4 is a fragmentar view, partly in section and partly in elevation, of the protector of my invention as applied to a grid lead passing through the stem of a vacuum tube.

In order that my invention may be more fully understood by direct reference to the drawings, I will first refer to Fig. 1 which shows the conventional three-electrode tube comprising vit reous envelope l which may be of a type of glass, for example, that will make a leak'proof seal with tungsten, either directly or through the medium of a glass which can be sealed to the main envelope and which will also seal to tungsten. The elements within the tube are illustrated diagrammatically and comprise an anode 2 mounted on a tungsten rod 3 passing through an anode seal 4 at the top of the envelope, a grid 5 supported within'anode 2 by a grid rod 6 also of tungsten and passing through a grid seal 1 in the side of the tube. The cathode of the tube may be a fairly heavy filament within grid 5, not shown, supported on filament support rods 8 sealed through a press 9 forming the end of the reentrant stem H) at the bottom of the tube. The center of the filament is supported by a central filament support H mashed in the press, but not generally passing therethrough. Such a tube is illustrative of tubes used for the generation or amplification of relatively high frequency oscillations and in the usual circuits for connecting the tube into operation to generate or transmit such oscillations, a grid lead 6 usually is maintained at an average potential negative to that of the anode lead 3. Under these conditions electrolysis of the glass in the seal 1 often occurs, and under certain conditions and at theend of the few hundred hours, for example, this seal may become discolored'and blackened, and then crack to admit air to the tube. As explained above it is my belief that the discoloration of the seal material is due to destructive electrolysis of the glass and that the eventual failure of the seal is due to the fact that the glass at this point has become changed in either chemical or physical characteristics, or both, to such an extent that the coefficient of expansion of the glass of the seal is no longer compatible with the coefficient of expansion of the tungsten lead 6.

I have found, however, that the failure of the seal 1 can be prevented, and the tube given a much longer life by surrounding the seal 1 with a conductive coating [5, this coating covering not only the seal 1 but also the glass of the envelope adjacent the seal. The coating I5 is also electrically connected to the lead 6. Coating [5 may be, for example, comprised of any conducting material dispersed in a binder or cement so that the coating can be dried into a hard conducting coat during the processing of the tube. The coating l5 may also be a thin coating of metallic foil applied to the tube and in intimate contact therewith, the main consideration being that the conducting material be positioned between the anode lead 3 and the grid lead 7 so that electrolysis currents passing through the envelope I from the anode lead to th grid lead will not be concentrated adjacent the seal 1 but will be spread over the glass beneath the coating nearest the anode. In other words. the electrolysis currents will be intercepted at a location spaced from the lead to which the conductive material is connected. In th case of Figs. 1 and 2 the coating has been applied as a metall c paste spread in circular form around the seal 1, contacting the lead 3 and extending outwardly over the envelope I. In this case the paste is dried and during the processing of the tube the metallic paste hardens into a solid, conducting coating intimately associated with the envelope material.

Life tests of a tube having the grid seal 1 protected as shown in Figs. 1 and 2, have shown that the life can be increased more than ten-fold. in

that many tubes without the seal protector dcscribed last only a few hundred hours under predetermined operating conditions whereas identical tubes operating under the same conditions and provided with my seal protector have lasted several thousand hours without failure. In the tubes with protected grid leads, the glass beneath the coating l5. closest to the anode lead 3. showed the discoloration of electrolysis. However this glass is removed from the seal area, and is not involved in any contact with any dissimilar materials.

The round protector shown in Figs. 1 and 2 is, of course, the easiest to apply, and in all cases where the electrolysis currents are not too high it is extremely satisfactory. However there may be cases where electrolysis currents may be still higher. and where there will be a high electrolysis current density in the glass beneath the edge of the coating l5 nearest the anode, to such an extent that failure might be expected due to a difference in expansion coefficients of the glass that has been electrolyzed, and the adjacent glass. Under these circumstances the type of coating shown in Fig. 3 may be adopted. In this case the coating is extended from the round shape, to an extended area It which has an upper edge I! which is normal to a plurality of paths from the anode lead. In this case there is no tendency for the current density along edge I! to be higher in any one point than any other, and the entire electrolysis current is spread out over the length of edge H. The currents density at any one point, therefore, is low. causes relatively little electrolysis of the glass, and there will be no tendency for the glass to become unduly electrolyzed because of the wide area of glass exposed to the electrolysis currents.

In other cases, the electrolysis currents may be distributed over a wider area of vitreous material, by making coating 15 of highresistance material.

In Figs. 1, 2 and 3 I have shown my invention as applied to the outside of the envelope I. However, it is obvious that my invention is equally effective whether positioned inside or outside, although I prefer to place the coating l5 on the outside of the envelope because it is easier to apply and there is no danger of introducing gas producing materials into the inside of the tube. However, there may be occasions where it is desirable to place the coating on the inside of the tube. Such an occasion is shown in Fig. 4 where the grid seal 1 is formed laterally in a special reentrant stem 20 carrying in the press the filament leads 8, and at the same time supporting the grid 5 through lateral uprights 6 sealed into the reentrant stem below the press 9. In this case only one of the grid leads passes through the stem 20 and it is this lead that is provided with the coating l5, although the dummy leads may also be protected by the coating l5 to prevent support failure if desired. In this case I may prefer, in order to prevent the introduction of gas producing materials into the inside of the tube, to form coating 15 of platinum foil fired into the glass around the seal, as is well known in the art, to make an intimate contact with the glass and the lead.

While I have shown my invention as being applied to the grid lead of a glass envelope threeelectrode tube carrying high frequency oscillations, it is obvious that my invention can be applied to the protection of leads in tubes having more than one grid. It is also obvious that tubes can be operated in such a manner, that other electrodes such as filament leads through stems, for example, in the tube are at an average negative potential with respect to remaining electrodes and that in this case the protecting device will be applied to the leads that are operating at the negative potentials. In an event the protection can be applied to any lead which is subject to destructive electrolysis of vitreous material around the lead. In this respect, therefore, my invention is applicable to tubes having metal envelopes, when vitreous seals are used for the leads.

I claim:

1. In an electron discharge device having a glass envelope containing as elements, a source of electrons, an anode, and a control electrode, leads to said elements passing through said envelope and sealed therethrough in a metal-toglass seal, and a conductive connection from said control electrode lead to a location on saic envelope spaced from said control electrode lead 2. Apparatus in accordance with claim 1 herein said connection is shaped to present a circumscribing edge about said control clec trode lead.

3. Apparatus in accordance with claim 1 wherein said connection is shaped to present an extended edge normal to current paths between the anode lead seal and said control electrode lead seal,

4. The method of protecting a metal-to-glass seal around an electrode lead in a gla s tube envelope when the glass adjacent the seal material is subject to destructive electrolysis which comprises covering said seal and adjacent envelope portions with a conductive coating in intimate contact with the material of said onvelope and seal and with the lead passing through the covered seal material and in contact therewith.

5. The method of preventing destructive electrolysis at metal-to-glass seals passing through a glass tube envelope which comprises conductively intercepting electrolysis currents in said envelope at a location on said envelope removed from said leads.

6. In an electron discharge device having a glass envelope containing a plurality of electrode elements wherein one of said electrode elements operates at a higher potential than another of said electrode elements, leads to said electrode elements passing through said envelope and sealed therein in metal-to-glass seals, and a conductive connection from the lead of the lower potential electrode element to a location on said envelope spaced from said lead. WINFIELD G. WAGENER. 

